Unfortunately, searching for portal textures when running Hammer with game set to Team Fortress 2 gives you only one thing:
http://i35.tinypic.com/2eltj76.png
Filtering by "portal" when running Hammer with game set to Portal obviously works fine.
And before you consider taking the textures from the portal gcf:
http://developer.valvesoftware.com/w...g_Game_Content
Ah, well. I was originally hoping for building an easter egg into pl_grassandcave where a tunnel when accessed with noclip or spectator would lead you into a Portal themed room, complete with companion cube and radio.